1. Field of the Invention
This application relates to a process for purification of a solution. More particularly, this application relates to a process for removing heavy metal contaminants from a solution using activated hydrotalcite or a mixture of activated hydrotalcite and activated alumina as an adsorbent.
2. Description of the Related Art
Historically, very little attention was addressed to the disposal of industrial waste, due in part to lack of governmental regulation, and, more importantly, probably due to an overall lack of knowledge or appreciation by industry of the long term effects of industrial waste on the environment. Now, however, due both to government regulations and corporate responsibility, much attention has been addressed to proper handling and disposal of waste materials classified as hazardous or toxic.
Such materials include organics, heavy metals, cyanides, etc. Treatments for such materials include physical, chemical, biological, thermal, etc., depending upon the type of pollutant. Treatment of solutions containing heavy metals usually fall into the categories of physical and chemical. Most popular are the chemical treatments which treat the heavy metals to form compounds which will precipitate such as, for example, treatment with alkalies such as caustic soda or lime to form hydroxide precipitates or treatments with sulfur-containing compounds to form sulfides or sulfates.
Such methods for treating heavy metal pollutants are discussed in "Electroplating Wastewater Pollution Control Technology", by George C. Cushnie, Jr. published by Noyes Publications, Park Ridge, N.J. in 1985, at pp 1-9, 30-37, and 48-54: in an article published in Chemical Engineering in August of 1984 at pp 51-64 entitled "Hazardous Waste Management: The Alternatives", by Mackie et al; and in an article published in Chemical Engineering in September, 1985 at pp 60-74 entitled "Wastewater Treatment", by Eckenfelder, Jr. et al.
While such treatments work well for their intended purpose, i.e., to remove heavy metals from a plating solution or the like, they, in turn, create new problems. Conversion of heavy metal compounds to precipitates by neutralization, for example, as described in the aforementioned articles, forms hazardous sludge which must, in turn, be disposed of in an environmentally acceptable manner.
Other treatments have also been discussed for the removal of heavy metals from solutions such as electroplating solutions. Such methods, which are mentioned in the aforesaid Mackie et al article on page 57, and the Eckenfelder Jr. et al article on page 71, include: ion exchange, using caustic soda instead of lime to increase resin selectivity; activated-carbon adsorption; and reverse osmosis and evaporation.
Manabe et al U.S. Pat. No. 4,458,030 discloses an adsorbent composition comprising 5 to 95 wt. % hydrotalcite and 5 to 95 wt. % activated carbon. The patentees point out that each of the materials have deficiencies as adsorption agents which are apparently overcome by a synergistic effect of using the materials in combination. The patentees state that the hydrotalcite useful in their adsorbent composition may be produced by reacting an aqueous solution of a water-soluble Mg compound and/or a water-soluble Zn compound, such as magnesium chloride, magnesium sulfate, zinc chloride, and zinc sulfate with an aqueous solution of a water-soluble Al compound such as aluminum chloride, aluminum sulfate and sodium aluminate together with an aqueous solution of an alkali such as sodium hydroxide, potassium hydroxide, ammonia, sodium carbonate, and sodium oxalate.
Hydrotalcite is a naturally occurring mineral having either the formula 6MgO.Al.sub.2 O.sub.3.CO.sub.2.12H.sub.2 O or Mg.sub.6p Al.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O. Known deposits of natural hydrotalcite are very limited and is not present as pure product. Rather it contains other minerals such as pennite and muscovite as well as potentially undesirable minerals such as heavy metals. Conventional practice recognizes that it is practically impossible to remove such impurities from a natural hydrotalcite.
Previous attempts to produce a synthetic hydrotalcite of higher purity have included adding dry ice or ammonium carbonate (a) to a mixture of magnesium oxide and alpha-alumina or (b) to a thermal decomposition product from a mixture of magnesium nitrate and aluminum nitrate and thereafter maintaining the system at temperatures below 325.degree. C. at elevated pressures of 2,000-20,000 psi. However, such processes are not practical for industrial scale production of synthetic hydrotalcite because of the high pressures employed. Furthermore, the use of such high pressures can form substances other than hydrotalcite such as brucite, boehmite, diaspore, and hydromagnesite.
Other processes for producing synthetic hydrotalcite are known. Ross and Kodama, in an article entitled "Properties of A Synthetic Magnesium-Aluminum Carbonate Hydroxide and Its Relationship to Magnesium-Aluminum Double Hydroxide Manasseite, and Hydrotalcite", published in 1967 in The American Mineralogist, at volume 52, pp 1036-1047, describe a process for producing a hydrotalcite-like material by titrating a mixed solution of MgCl.sub.2 and AlCl.sub.3 with NaOH in a CO.sub.2 free system and then dialyzing the suspension for 30 days at 60.degree. C. to form a hydrated Mg-Al carbonate hydroxide. The mineral product is associated with the formula Mg.sub.6 Al.sub.2 CO.sub.3 (OH).sub.16.4H.sub.2 O and has the properties of manasseite and hydrotalcite.
Reichle U.S. Pat. No. 4,458,026 discloses novel catalyst materials produced as a preparation of Mg/Al carbonate hydrotalcite which involves the addition of mixed magnesium/ aluminum nitrates, sulfates, or chlorides as an aqueous solution to a solution of a stoichiometric amount of sodium hydroxide and carbonate at about 25-35.degree. C. while stirring over a period of several hours to produce a slurry. The slurry is then heated for about 18 hours at about 50.degree.-200.degree. C. (preferably 60.degree.-75.degree. C.) to allow a limited amount of crystallization to take place. After filtering the solids, washing, and drying, the dry solids are recovered.
Kamura et al U.S. Pat. No. 3,650,704 also describes the preparation of synthetic hydrotalcite by adding an aqueous solution of aluminum sulfate and sodium carbonate to a suspension of magnesium hydroxide. The suspension is then washed with water until the presence of sulfate radical is no longer observed. The suspension is then heated at 85.degree. C. for three hours and dried. The magnesium component of the starting material is reported as any member of the group consisting of magnesium oxide, magnesium hydroxide, magnesium carbonate, and water-soluble magnesium salts, e.g., such as mineral acid salts including magnesium chloride, magnesium nitrate, magnesium sulfate, magnesium dicarbonate, and bittern.
In Misra U.S. patent application Ser. No. 788,853, cross-reference to which is hereby made, the production of a unique synthetic hydrotalcite is described and claimed wherein an activated magnesia is reacted with an aqueous solution containing ions of aluminate, carbonate, and hydroxyl to form hydrotalcite in high yield and of high purity. The purpose of using activated magnesia was to insure formation, by the reactants, of hydrotalcite rather than other materials which would lower both the yield and the purity of the synthetic hydrotalcite.